1. Field of the invention
The present invention relates to a resist composition and a process for the formation of resist patterns using the same. More particularly, the present invention relates to a resist composition which can be exposed to any patterning radiation having a short wavelength such as excimer laser light and also can be subjected to development using an aqueous basic solution, after said resist composition is coated and exposed to a patterning radiation, and a process for the formation of negative-working resist patterns using such a resist composition. It should be noted that, as will be appreciated from the following detailed description of the present invention, the resist composition according to the present invention is distinguished from the conventional chemical amplification resists in view of the structure and functions of the polymeric material constituting the resist, and the composition can provide fine resist patterns having no swelled portion without suffering from any adverse influences due to varied environmental conditions in the resist process. Further, the thus produced resist patterns can provide a practically usable sensitivity and an excellent resistance to dry etching, because the structure of the acrylate or methacrylate (hereinafter referred to as "(meth)acrylate") copolymers constituting the resist has been modified for the purpose of obtaining such surprising effects. Accordingly, the present invention can be advantageously utilized in the production of semiconductor devices such as semiconductor integrated circuits, for example, LSIs, VLSIs, ULSIs and other devices, using a lithographic process.
2. Description of the Related Art
Recently, in the production of the semiconductor integrated circuits, the degree of integration thereof has been notably increased and accordingly LSIs and VLSIs have been produced on a commercial scale. The minimum line width of the circuit patterns in these devices approaches the sub-half micron or quarter micron order. In other words, in the production of these high performance devices, it is required to provide an established fine fabrication technology.
In the field of lithography, to satisfy the above requirements, an approach in which ultraviolet (UV) radiation as an exposure source is shifted to shorter wavelengths in the far or deep ultraviolet region has been suggested, along with research for providing new exposure devices provided with a light source capable of emitting such a short wavelength radiation in the deep ultraviolet region. In addition, there has been urged to find a novel resist material showing a less absorption to the above-mentioned shorter wavelength radiation, good sensitivity and high resistance to dry etching.
It should be noted that, recently, an exposure method using an argon fluoride (ArF) excimer laser having a wavelength of 193 nm as an exposure source has been developed to take an initiative in a next generation of the exposure technology in the production of semiconductor devices. Further, in view of such a tendency, it is an urgent necessity to find a resist material capable of providing, good sensitivity and resolution in such short wavelength regions. The resist material must satisfy the requirement concerning transparency at the wavelength of 193 nm, in addition to a good performance which is generally required in a conventional resist material. The conventional aromatic ring-containing photoresists, typical examples of which include a novolak resin and polyvinyl phenol resin, however, can not be used in the above-mentioned exposure method, because they are opaque at the wavelength of 193 nm due to their strong absorption at 193 nm. Alternatively, it is thought that introduction of a reaction mechanism of chemical amplification in the resist material would provide a highly increased sensitivity, however, the prior art chemical amplification-based resist materials are not considered to be satisfactory, because they still have the problem concerning their high susceptibility to the varied environmental conditions in the resist process. The problems of the prior art chemical amplification-based resist materials are as follows.
B. Reck et al., Polym. Eng. Sci., 29, 960, (1989) teach a chemical amplification negative resist material in which the chemical amplification is based on an electrophilic aromatic ring-substitution reaction and which material contains a polyvinylphenol or novolak resin as a base resin. In this resist material, the substituted benzyl acetate as a latent electrophilic compound is used in combination with an onium salt as a photo acid generator (PAG). In this resist material, an acid is released from the PAG upon exposure of the material, and then the acid hydrolyzes the substituted benzyl acetate, thereby generating benzyl carbonium anions. Thereafter, the thus generated carbonium anions, upon post-exposure baking (PEB) of the exposed resist material, electrophilically attack an aromatic ring of the base resin, thus producing a crosslinked product thereof. In the exposed areas in which the crosslinked product was produced, an increase in the molecular weight is observed, i.e., notable differences in the solubility in an aqueous basic solution between the exposed areas and the nonexposed areas which are sufficient to obtain negative resist patterns are generated. The resist composition suggested by B. Reck et al. is satisfactory, if it is wished to obtain the resist patterns with a very high sensitivity, after exposure to krypton fluoride (KrF) excimer laser having a wavelength of 248 nm, slightly longer wavelength than ArF excimer laser, as an exposure source. However, when the ArF excimer laser is used in place of the KrF laser, it becomes impossible to obtain the resist patterns, because said resist composition has a transmittance of substantially zero at the wavelength of 193 nm. In addition, since the patterning process is based on a mechanism of chemical amplification, the resist material is liable to be adversely affected by the varied environmental conditions in the resist process. Accordingly, the resist material suffers from the unavoidable drawbacks such as variation in the size of the resulting resist patterns and pattern defects, for example, corrosive cutoff or curved edges, between the resist patterns and the underlying substrate. This means that the resist materials have to be used and fabricated with the greatest care.
On the other hand, C. J. Dubois et al., 8th Int. Conf. Elecron and Ion Beam Sci. Technol., Seattle, 303 (1978) teach an acrylic crosslinkable chemical amplification negative resist material in which a copolymer of methylmethacrylate and 2,3-epithiopropylmethacrylate is used in combination with a diazonium salt. The resist material shows an excellent transparency to both the KrF and ArF excimer laser, however, its resistance to dry etching is poor. Therefore, generally, the resist material is not used alone or as a single resist layer, however, it is used in combination with another resist layer having a good resistance to dry etching.
As it can be seen from the above descriptions, the prior art chemical amplification resist materials cannot be utilized in the formation of the resist patterns in which the ArF excimer laser is used as an exposure source, and in addition to this drawback, they suffer from the problems that defective resist patterns are unavoidably formed due to variation of the environmental conditions in the resist process and that it is difficult to simultaneously attain both a good transparency and a good resistance to dry etching. Further, the conventional and well-known negative-working photoresists consisting of cyclic polyisoprene-bisazide, for example, "KTPR" commercially available from Eastman Kodak Company, "OMR" commercially available from Tokyo Oka Kogyo Co. and others, are insufficient to avoid swelling in the resulting resist patterns, because the resist patterns are formed upon development of the exposed resists with an organic solvent as a developer. The swelled resist patterns mean that the fine fabrication of the target substrate is limited to an insufficient level. Furthermore, another type of the conventional resist material, namely, X-ray resist consisting of the copolymer of allyl methacrylate and 2-hydroxyethyl, has no sensitivity in the ultraviolet region as will be appreciated from its reaction mechanism, and also its resistance to dry etching is poor. It is, therefore, desired to further improve this resist material.
The requirements in the prior art resist materials are various, and are contrary to each other. However, as is disclosed in Y. Kaimoto et al., Advances in Resist Technology and Processing, IX Proc. SPIE, 1672, 66-73 (1992), they found that all the requirements can be satisfied if the resist material is designed as a chemical amplification resist and it is constituted from a copolymer of adamantyl methacrylate and t-butyl methacrylate as a base resin and triphenylsulfonium hexafluoroantimonate as a photo acid generator. Surprisingly, since it uses adamantyl methacrylate as one monomeric component of the base copolymer, the resist material can exhibit a high transparency at both the wavelengths of 193 nm and 248 nm, and at the same time, a good resistance to dry etching. However, one drawback of this resist material is that the resist material is designed to form positive-working resist patterns and accordingly, using this resist material, negative resist patterns cannot be obtained.